The present invention relates to a lens actuator, and more specifically to a lens actuator for an optical head of an optical disc drive. In this specification, optical disc drives include not only an optical disc drive dedicated exclusively to reading of optical discs but also a magneto-optical disc drive capable of reading and writing in magneto-optical discs by means of magnetic field modulation or optical pulse modulation.
An optical disc drive has been widely used as a high-capacity external storing apparatus for a computer or the like. Toward the advent of the forthcoming era of multimedia, there is an increasing demand for higher density recording. One measure which the optical head may adopt for increasing the recording density of the optical disc drive is to reduce the spot diameter of a laser beam. The spot diameter of a laser beam is proportional to xcex/NA, where xcex represents the wavelength of the source laser beam and NA represents the number of apertures of the object lens in the head. Therefore, the beam spot diameter is reduced by decreasing the wavelength of semiconductor laser, or by increasing the NA of the object lens in the head.
However, decrease of the wavelength of semiconductor laser, namely utilization of blue semiconductor laser, still encounters problems with respect to cost, output efficiency, temperature stability and so on, posing difficulty in practical utilization at the present time.
On the other hand, the option of increasing the NA of the object lens in the head is also problematic because if the goal is to be achieved by a single lens, the radius of curvature of the lens needs to be rendered small. Then, tolerances with respect to the lens surface curvature and dimensions in production of the lens have to be accordingly small, making difficult to manufacture an appropriate lens. Further, errors increase with respect to tilt and eccentricity between the surfaces of a lens (between the incidence and the exit surface), tilt and eccentricity in the lens itself, coma caused by tilt of the disc (proportional to NA to the third power ), and spherical aberration caused by inconsistent thickness of the disc (proportional to NA to the fourth power). These errors make it difficult to carry out recording onto the disc as well as reading from the disc.
JP-A-8-221790, for example, proposes an optical pickup device for solving the above-described problem. As shown in FIG. 11, the optical pick up device disclosed in this publication comprises an object lens system including two separate lenses, i.e., a first object lens l1 and a second object lens l2 each having a relatively small NA. The second object lens l2 is held by a tube m, whereas the first object lens l1 is held by a slider s supported by the tube m via an elastic member b in between. When the disc is driven, the slider s slidably contacts a disc surface or floats off the disc surface via an air film (air bearing) in between.
With the optical pickup device having the above structure, the overall NA of the object lens system increases due to the combination of the two lenses each having a relatively small NA, providing basis for meeting the demand for recording density increase. Further, the problem associated with the single-lens approach, i.e., the problem arising from the decreased radius of curvature in the lens is reduced. Still further, according to the description of JP-A-8-22179, the wave aberration is reduced since the slider s follows the disc surface.
In the case where the slider slidably contacts the disc, the slider will follow the disc surface almost perfectly and it will be possible to reduce the wave aberration. In this case, however, there is a problem that the disc will be scratched. For this reason, the slider should preferably be floated off the disc surface for protecting the disc surface or for maintaining the strength of the slider. However, if the slider floats off the disc via an air bearing, the following problems may arise.
Specifically, the reduction in the wave aberration achieved by the object lens system including the first object lens l1 and the second object lens l2 as proposed in the above publication can be achieved only on the assumption that the slider changes its posture following the disc, thereby reducing fluctuations in the distance and in the inclination (tilt) of the first object lens relative to the disc. If the slider floats via the air bearing which is formed by an air stream flowing between the turning disc and the slider, the amount of floatation and the amount of tilt of the slider relative to the disc actually fluctuate due to various factors such as temperature, relative speed between the disc and the slider (which varies depending on the radial location of the track on the disc), elastic load applied to the slider, machining errors in the disc and the slider, and so on. If the goal of high-density recording and reading is to be achieved by increasing the NA of the object lens system, tolerances allowed for the optical head are extremely small. Therefore, the structure in which the object lens system is simply provided by the first object lens and the second object lens with the first object lens held by the slider which follows the disc, the data recording/reading characteristics may deteriorate by fluctuations in the amount of floatation and tilt of the slider relative to the disc caused by the various factors mentioned above.
It is therefore an object of the present invention to appropriately reduce fluctuations in the amount of floatation and/or in the amount of tilt of the lens held by the slider relative to the disc in an optical disc drive of the type wherein a lens held by the slider which floats off the disc when the disc drive is operating is combined with another lens for increasing the NA of the object lens system in an optical head.
According to the present invention, a lens actuator of an optical head for an optical disc drive is provided which comprises a two-dimensional actuator supported by a carriage via a spring and driven to displace in focus directions and track directions, a second lens including at least one lens held by the two-dimensional actuator, a slider supported by the two-dimensional actuator and floated off a turning optical disc via an air bearing; and a first lens including at least one lens and held by the slider, whereby a laser beam is focused on the optical disc by means of an object lens system which includes the first lens and the second lens, wherein the slider is supported by the two-dimensional actuator via a support having a predetermined spring constant in the focus directions while also having freedom to follow tilting of the optical disc.
Since the optical head lens actuator according to the present invention also utilizes a plurality of lenses for increasing the overall NA of the object lenses, it is possible to increase the recording/reading density. Of the above lenses, the first lens including at least one lens closer to the disc is held by the slider which is floated off the disc via an air bearing. Since the slider basically follows the tilt of the disc surface, it is possible to reduce the wave aberration.
Thickness inconsistency and warping of the disc cause dynamic fluctuations of the disc surface height and tilt when the disc is turning. Therefore, in order to allow the slider to follow the above dynamic fluctuations of the disc surface for maintaining the amount of floatation and tilt with respect to the disc surface as constant as possible, the load applied against the disc must be controlled. According to the present invention, the two-dimensional actuator displaceable in the focus directions and in the track directions supports the slider via the support which has a predetermined spring constant. Thus, it is possible to dynamically control the load of the slider applied against the disc by moving the two-dimensional actuator in the focus directions. The control basically utilizes a focus error signal which reflects the location of the slider, as a carrier for the first lens, from the recording surface of the disc. The focus error signal is used as a feedback signal in controlling the driving electric current for the two-dimensional actuator in the focus directions. In this way, once the fluctuations of the floatation are reduced by controlling the load of the slider applied against the disc, the fluctuations in the amount of tilt of the slider relative to the disc are automatically reduced, because according to the present invention the support holding the slider on the two-dimensional actuator has freedom to follow the tilt of the disc, though having a predetermined spring constant in the focus directions.
When the control to the load applied onto the slider toward the disc is performed as described above by controlling the driving electric current for the two-dimensional actuator in the focus directions in response to a focus error signal as a feedback signal, the control can dynamically follow the fluctuations of the disc surface if the following conditions are met. Specifically, the inventors of the present invention found it preferable that the support for supporting the slider on the two-dimensional actuator has a resonance frequency higher than the resonance frequency of the spring for supporting the two-dimensional actuator on the carriage and lower than the resonance frequency of the air bearing formed between the slider and the optical disc. This will be further described later.
According to a preferred embodiment, the two-dimensional actuator is supported by the carriage via a support spring.
According to another preferred embodiment, the support for supporting the slider on the two-dimensional actuator holds the slider on two opposite sides thereof.
According to a further preferred embodiment, the support for supporting the slider on the two-dimensional actuator is spiral.
Other features and advantages of the present invention will become clearer from the following detailed description to be presented with reference to the accompanied drawings.